In conventional medical diagnostic imaging the object is to obtain an image of a patient's internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5 percent or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element. Crossover of light from one fluorescent screen to an emulsion layer on the opposite side of the support of the radiographic element results in a significant loss of image sharpness. For medical diagnostic imaging, film contrast typically ranges from about 1.8 to 3.2, depending upon the diagnostic application. Crossover is minimized. In the highest speed diagnostic dual-coated radiographic elements, those employing spectrally sensitized tabular grain emulsions, crossover typically can range up to about 25% in the absence of other crossover control measures. In fact, it is common practice to add processing solution decolorizable dye particles to reduce crossover to near zero. X-radiation exposure energies vary from about 25 kVp for mammography to about 140 kVp for chest X-rays.
Examples of radiographic element constructions for medical diagnostic purposes are provided by Abbott et al U.S. Pat. Nos. 4,425,425 and 4,425,426, Dickerson U.S. Pat. No. 4,414,310, Kelly et al U.S. Pat. Nos. 4,803,150 and 4,900,652, Tsaur et al U.S. Pat. No. 5,252,442, and Research Disclosure, Vol. 184, August 1979, Item 18431.
Portal radiography is used to provide images to position and confirm radiotherapy in which the patient is given a dose of high energy X-radiation (from 4 to 25 MVp) through a port in a radiation shield. The object is to line up the port with a targeted anatomical feature (typically a tumor) so the feature receives a cell killing dose of X-radiation. In localization imaging the portal radiographic element is briefly exposed to the X-radiation passing through the patient with the shield removed and then with the shield in place. Exposure without the shield provides a faint image of anatomical features that can be used as orientation references near the target (e.g., tumor) area while the exposure with the shield superimposes a second image of the port area. The exposed localization radiographic element is quickly processed to produce a viewable image and confirm that the port is in fact properly aligned with the intended anatomical target. During the above procedure patient exposure to high energy X-radiation is kept to a minimum. The patient typically receives less than 20 RADs during this procedure.
Thereafter, before the patient is allowed to move, a cell killing dose of X-radiation is administered through the port. The patient typically receives from 50 to 300 RADs during this step. Since any movement of the patient between the localization exposure and the treatment exposure can defeat the entire alignment procedure, the importance of minimizing the time elapsed during the element processing cycle is apparent. Reducing this time by even a few seconds is highly beneficial.
A second, less common form of portal radiography is the verification of the location of the cell killing exposure. Again, the object is to record enough anatomical information to confirm that the cell killing exposure was properly aligned with the targeted anatomy.
It is appreciated that the large differences in exposure times that distinguish localization and verification imaging have up to the date of this invention precluded the successful use of a single portal radiographic element to serve both applications.
Both localization and verification portal imaging have suffered from very poor image quality. Anatomical features are often faint, barely detectable or even non-detectable. This has severely restricted reliance on portal radiography.
Although excellent radiographic imaging capabilities have been realized in medical diagnostic imaging, there are fundamental differences in the imaging physics that distinguish and render nonanalogous diagnostic and portal radiographic imaging. In diagnostic imaging X-radiation photon energy of up to 140 kVp is in part absorbed within the patient and in part passed through to be absorbed in a fluorescent intensifying screen to generate light that exposes the radiographic element.
In portal imaging the multi-MVp X-radiation in part passes through the patient unabsorbed and is in part absorbed creating a secondary electron emission. A front metal intensifying screen is relied upon to intercept and absorb the secondary electron emission. This lowers minimum density and significantly enhances image sharpness. Image intensification (raising maximum density and contrast) is achieved by absorbing X-radiation and transmitting to the radiographic element electrons that are thereby generated. The much higher capability of the radiographic element to absorb electrons as compared to X-radiation produces image intensification. Besides supplying electrons that are relied upon to expose the radiographic element, the front intensifying screen further contributes to image sharpness by transmitting to a much lesser extent electrons generated by obliquely oriented (i.e., scattered) X-radiation that it receives.
In addition to the front metal intensifying screen, which is always present, a back metal intensifying screen can be employed to provide an additional source of electrons for radiographic element exposure.
Recognizing the imaging deficiencies of conventional portal radiography, Sephton U.S. Pat. 4,868,399 suggests replacing a conventional dual-coated, low contrast, and low crossover radiographic element (e.g., Kodak TL.TM.) sandwiched between front and back metal intensifying screens with the combina- tion of a conventional high (4-8) contrast graphic arts or lithographic line film (e.g., Kodaline 2586.TM.) and a fluorescent intensifying screen.
The Sephton portal radiographic imaging assembly failed to achieve acceptance. It suffers a number of deficiencies that place it at fundamental variance with user needs. These include (1) a greater susceptibility to false image information (e.g., the mistaking of dust or debris for anatomical information), (2) incompatibility with radiographic processors, particularly short processing cycles, and (3) an inability to accommodate a variety of exposure applications (Sephton reports only localization imaging). In addition, Sephton follows conventional diagnostic imaging practices in urging "the closest possible contact between the lead, the fluorescent layer, and the emulsion".